The development of Engineering Recommendation G74

This paper summarises the outcome of a great deal of work done collaboratively within the UK Electricity Industry to develop improved methods for the calculation of short circuit currents in transmission and distribution networks. This was done against a background of a number of limitations associated with IEC 909, the only internationally published standard method. ER G74 sets out a procedure which requires a more detailed assessment of short circuit currents with a fuller representation of system pre-fault conditions and taking account of dynamic load fault circuit contribution. It is hoped that as experience is gained, particularly with the measurement of LV motor load fault contribution, improvements can be made to the procedure. It is also hoped that the work done so far will go a long way towards influencing a revision of IEC 909 to make it more appropriate to interconnected transmission and distribution networks and to computer simulation techniques